Removing carbon and carbonaceous deposits from heat exchanger equipment



1366- 1951 s. D. LAWSON REMOVING CARBON AND CARBONACEOUS DEPOSITS FROM HEAT EXCHANGER EQUIPMENT Filed Jan. 20, 1947 w E m S m w R L W M I a M 1 mm A m ms I H G 1 5 m m T H n 1 L S C B A X n Y W u a ii iiwi n A I n u D m r m l I. K. I t. A: C mm E m S my 4 S K 2 T m l T u m i t E 2 A 1 CC M l m m 5 M r mu m v u T 7 w" A E. W 6 *R r| :1... m" lb 1 Patented Dec. 4, 1951 REMOVING CARBON AND CARBONACEOIIS DEPOSITS FROM HEAT EXCHANGER EQUIPMENT Shelby Dayton Lawson, Bartlesville, kla., assignor to Phillips Petroleum Company, a corporation of Delaware Application January 20, 1947, Serial No. 723,040

3 Claims. 1

The present invention relates to the removal of carbon and carbonaceous deposits from heat exchanger equipment. More particularly the present invention relates to a method and apparatus for the removal of coke or carbonaceous material from tubes and other portions of equip-- ment employed in heating of'hydrocarbon fluids or in waste heat exchangers wherein hot hydrocarbon fluids are cooled by indirect heat exchange with steam or other fluid.

Heat exchanger equipment which is used to heat or cool a stream of hot hydrocarbon fluid eventually accumulates carbon, coke or carbonaceous deposits. This is particularly true where the temperature conditions and type of hydrocarbon are such that thermal cracking of some of the hydrocarbon components may occur. In tubular types of heat exchangers, these deposits form on the inner or outer tube surfaces and thus may reduce the available cross-sectional area of the tubes and/or greatly reduce the heat transfer efliciency through the walls of the tubes. Therefore, in order to maintain efllcient operation, these deposits must be frequently removed from the tube surfaces. It is common practice to accomplish this by taking the equipment apart and removing the deposits by manual means. This procedure requires considerable labor and loss of service of the equipment for a considerabletime.

In order to avoid the necessity for disassembly of equipment and manual removal of carbon, various expedients have been resorted to in order to remove the deposits. Among such methods it has been proposed to remove the deposits by combustion or oxidation with air. Such methods have been found diflicult to control and have resulted in the development of excessive temperature with consequent damage to the heat exchanger tubes.

In accordance with the present invention a method and apparatus are provided which will enable carbonaceous deposits to be removed in a simple and controlled manner from heat exchanger equipment. The invention comprises a sequence of operations and equipment for carry- In accordance with the present invention carbon is removed from within a heat exchanger in a sequence of operations and utilizing apparatus which will prevent the development of excessive temperatures and protect the equipment in the event high temperature should tend to occur. In carrying out the process oxygen-containing gas is introduced into the carbonized exchanger under combustion conditions. The combustion takes place externally of the tube bundle and on the outer tube walls where hot hydrocarbons have been cooled by passage of cooling fluid through the tubes. Where hydrocarbons are preheated or cracked the flow is usually through tubes in a furnace and carbon is deposited within the tubes. In using oxygen to remove these deposits, the combustion is usually carried out at atmospheric or higher pressures and in order to prevent the tubes from overheating by combus tion at the surface thereof, and also to prevent collapse or undue stresses between tubes, tube supports and shell, steam is introduced into the tubes or surrounding the tubes, as the case may be, in order to equalize pressures between the combustion zone and the zone in which coinbustion is not carried out. For example, where combustion is carried out externally of the tubes, steam under pressure is introduced and maintained in the tubes during combustion. The pressure is regulated to be at least equal to that in the combustion zone, and may also be some-' what greater. Where carbon is to be removed from the outer surfaces of the tubes in a tubular heat exchanger as referred to below, it is frequently desirable to introduce a spray of water adjacent the shell head for the purpose of cooling it during combustion to prevent overheating, and additionally to generate steam which will act as a diluent and temperature controlling medium. The drawing illustrates the type of apparatus to be used and the procedure to be followed, in a diagrammatic manner. In the drawing, catalyst chamber l contains a suitable cracking catalyst such as bauxite, activated clay, or the like. A hydrocarbon feed such as gas oil, preheated to conversion temperature, say in the range of 850-1100 F., diluted if desired, with steam or other diluent, is introsteam is passed. The hydrocarbon products cooled to .a desired temperature, say 700 F. are removed via lines 5 and 6 and valve I to a fractionation system for the recovery of desired products. After the catalyst has become wholly or partially spent by deposition of carbonaceous material during the conversion, the catalyst is reactivated by introducing a steam-air mixture through line 2 and initiating combustion in the bed. The combustion products are removed through 3 and pass in heat exchange with lower temperature steam in exchanger 4 and out through lines 5 and 8 and valve 9 to vent stack. Ordinarily a pair of catalyst chambers each with a heat exchanger is provided so that while one is on conversion, the catalyst in the other may be reactivated.

When an undesirable quantity of carbon has been deposited on the exchanger tubes it will be necessary to suspend operation of the heat exchanger in order to effect the burn-out operation described below. In this operation valve A is closed and valve B is opened. The operation of these valves may be manual or automatic and preferably the closing of valve A will automatically open valve B. Closing of valve A stops water or steam circulation to the heat exchanger bundle. Valve B controls air flow to operate valve C which opens with air pressure on a diaphragm, but valve C may be manually or otherwise operated. When valve A is closed, valve D is opened. This drains all of the water from the heat exchanger and vapor line into accumulator 10. Steam flows from the steam drum ll through a small line I2 and valve G into the heat exchanger to maintain a desired pressure within the tubes to balance any pressure within the shell of the exchanger during the burn-out. F is a check valve permitting flow into the-steam drum during conversion or regeneration but cutting oiI flow in the reverse direction during the burn out operation. Valve G may be left in fixed position during steam generation as well as during the burn-out operation. Valve K is a control valve which will maintain the desired pressure in the system during the burn-out. Valve C controls flow of water to spray nozzle I3 which throws a cone spray of water on the heat exchanger shell head to keep it from overheating. Valve E is then opened to permit flow of air into the heat exchanger for combustion of coke etc. on the tube walls and elsewhere in the exchanger. The air is preferably preheated to combustion temperatures and may be admixed with steam as a diluent to control burning temperatures. Air flow is continued until the desired amount of carbon is removed while at the same time the water spray on the shell head continues to operate. In this way steam is formed in the exchanger which also serves to control temperatures therein.

At the end of the combustion period valve E is closed, shutting off air flow. Water valve M is then opened and water .is introduced to quench the exchange bundle and cool it to a desired tem perature as indicated. by the thermocouple T, say 400 F. or below. Valve D is then closed and valve A is opened while valves B and C are closed. This resumes water or steam circulation through the exchanger as the conversion regeneration cycle is resumed. A temperature controller 14 connected to thermocouple T may be provided to prevent opening of valve A until the bundle i cooled to the desired temperature;

The system as described may be wholly or partially manually controlled or may be wholly or partially automatic. Preferably one steam drum is connected to two exchangers, so that steam is being produced continuously while one or the exchangers is on bum-out.

Example Virgin gas oil mm West Texas crude having an A. P. I. gravity of 36 was charged to a catalyst chamber of the type described herein at the rate of 250 barrels per hour. The inlet temperature was 1020 F. at 85 pounds per sq. in. gauge. and the outlet temperature was 940 F. at a pressure of pounds. After each six hour process period, the carbon deposited on the bauxite catalyst was removed by combustion with an airsteam mixture. During the cracking or the gas oil, the reaction products were cooled by heat exchange with water in a tubular heat exchanger to a temperature of 470 F. During regeneration, the spent gases were cooled in the same way to a temperature of about 700 F. from a chamber exit temperature of 1350 F. After a period of 22 days, it was found that a substantial amount of carbon had been deposited on the outside walls of the exchanger tubes.

The carbon deposit on the tubes was then removed in accordance with the present invention using air introduced at the rate of 26,500 cubic feet per hour for 48 hours. During this time low temperature steam was introduced into the tube to equalize the pressure and keep the tube wall temperature from getting too high to injure the tubes. In this way the heat exchanger outlet temperature was maintained at a maximum of 1000" F. during the burn-out. At the conclusion of the 48 hour period, the tube bundle was quenched by introducing water at the rate of 50 gallons per minute and the exchanger was ready for use in the cycle. The deposited carbon was substantially removed during the operation.

I claim:

1. A process for the removal of carbonaceous deposits from the outer walls of heat exchange tubes in a tube and shell heat exchanger, said deposits resulting from an operation in which said outer walls have been in contact with a hydrocarbon fluid at a temperature of from 850 to 1100 F. and in which a heat exchange medium has passed through said tubes, which comprises contacting said outer walls with air at combustion temperature and at a pressure above atmospheric, thereby removing all of said carbonaceous deposits by burning 01! same, during said burning introducing into said tubes low temperature steam at a pressure and in such amount that pressure is equalized outside and inside said tubes and overheating of said tubes is prevented, during said burning spraying water on the inner wall of said shell around its periphery, thereby preventing overheating of said shell, and upon termination of said burning cooling said tubes by introducing water into a combustion zone formed by said shell and said tubes wherein said burning has taken place.

2. A process for the removal of carbonaceou deposit from the outer walls of heat exchange tubes in a tube and shell heat exchanger, said deposits resulting from an operation in which said outer walls have been in contact with a hydrocarbon fiuid at a temperature within the range of 850 to 1100 F. and in which a heat exchange medium has passed through said tubes, which comprises contacting said outer walls with an oxygen-containing gas at combustion tem perature and at a pressure at least as high as atmospheric, thereby removing all of said carbonaceous deposits by burning, spraying water 7 said shell and forming steam as a temperature controlling medium.

8. A process for the removal of carbonaceous deposits from the surface of indirect heat exchange equipment enclosed within an outer shell. said deposits resulting from an operation in which said surface of said heat exchange equipment has been in contact in a first zone with a hydrocarbon fluid at a conversion temperature for said fluid and .in which the opposite side of said heat exchange surface is contacted in a second zone with a heat exchange medium, which comprises introducing into said first zone an oxygen-containing as under combustion conditions at a pressure at least as high-as atmospheric, thereby removing said carbonaceous deposits b burning.

spraying water on the peripheral inner wall of said outer shell during' said burning, thereby preventing overheating of said shell and forming steam as a temperature controlling medium.

SHELBY DAYTON LAWSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 725,363 Powter Apr. 14. 1903 1,163,303- Allabor Dec. '1. 1915 1,928,085 Wyndham et a1. Sept. 26, 1933 2,028,250 Rossner Jan. 21, 1936 2,057,441 I McAllister Oct. 13. 1936 2,289,351 Dixon et al'. July 14, 1942 

